A. Field of the Invention
The present invention relates to a torque converter used for transmitting torque from a crank shaft of an engine to a manual transmission, and in particular, the present invention relates to a torque converter having both a lock-up clutch and a disengaging clutch mechanism.
B. Description of the Related Art
Recently, automotive manufacturers have begun using a torque converters with manual transmissions. Torque converters, in general, include a converter housing with an impeller formed on an inner surface of the housing, a turbine rotatably supported within the housing and a stator rotatably supported within the housing between the turbine and the impeller. The converter housing is typically formed with power input front cover which is connected to the crankshaft of an engine. One such torque converter is disclosed in Japanese Unexamined Utility Model No. SHO/64/31355, for example. In such a torque converter, a disengaging clutch couples the turbine and a power input shaft of a transmission allowing the torque converter to be manually engaged and disengaged to accommodate use with a manual transmission. The disengaging clutch is connected to one surface of the turbine. In other torque converters, a lock-up clutch may also be provided to directly couple the power input front cover with the turbine.
When starting a vehicle equipped with a torque converter and a manual transmission, torque is applied from the power input front cover to an impeller of the torque converter, causing the impeller to urge fluid within the torque converter to move toward the impeller causing the turbine to rotate. As a result, the vehicle starts to move smoothly. When the speed of rotation of a power input shaft of the transmission reaches a predetermined level, the lock-up clutch engages so that the torque is transmitted directly from the power input front cover to the power input shaft of the transmission. Since the torque is mechanically transmitted, the vehicle runs with greater fuel efficiency. When changing gears, the disengaging clutch is operated and the gears of the manual transmission may be shifted.
One drawback to the above-described configuration is that the torque converter has a rather complicated configuration in that the disengaging clutch and the lock-up clutch require separate mechanisms for engagement and disengagement. Such a configuration is complicated and expensive to manufacture.
There are torque converter configurations where the lock-up clutch includes a piston capable of being urged into press-contact with the inner surface of the front cover of the torque converter housing for engagement of the lock-up clutch. The lock-up clutch may further include a plurality of coil springs coupling the piston with the power output element. Another prior art torque converter includes a friction resistance generating mechanism which generates friction resistance when torsional vibration is applied to a lock-up clutch.
In such lock-up clutches, torsional rigidity of the coil spring must be reduced in order to effectively dampen torsional vibration under small load conditions. It is possible to reduce the wire diameter of the spring coil in order to reduce the torsional rigidity of the coil spring, however reducing the wire diameter also causes a corresponding reduction in the overall torque transmission capability of the torque converter. Thus, the coil diameter of the coil spring is usually kept at a relatively large diameter so that capacity of torque transmission is retained. However, the torsional rigidity of the torque converter is increased as a result. Further, the large diameter coil spring occupies a larger space, thus preventing downsizing the lock-up clutch.
In order to reduce the transmission from developing abnormal sounds like clattering sound and internal indistinct sounds during ordinary driving conditions, it is desirable for the resonance frequency of the torque converter to be reduced to a level generally equal to or below the idle speed of the engine associated with the torque converter and manual transmission.
In the above-described prior art torque converter and the vehicle in which it is used, under the ordinary driving conditions where the lock-up clutch operates, the power transmission system may be divided into a power input portion and a power output portion which are separated by the coil spring of the lock-up clutch therebetween. The dynamic characteristics of the power input portion and the power output portion must be evaluated in order to design a torque converter which provides adequate vibration reduction and reliable torque transmission. Ideally, in a power transmission system, it is necessary to sufficiently increase a ratio of moment of inertia of the power output portion to the power input portion to reduce the resonance frequency to be equal to or less than the idle speed of the engine. However, when the lock-up clutch is disengaged, it is desirable to reduce an inertial mass of a power output portion so as not to exert an adverse effect to components like clutches.
A centrifugal mechanism has been used to couple and uncouple an annular weight to the lock-up clutch. In such a design, as the speed of rotation of the turbine increases, centrifugal force works so that the annular weight is coupled to power output elements including the turbine. Consequently, the ratio of moment of inertia of the power output portion to that of the power input portion becomes larger, and the resonance frequency can be reduced to a low frequency region in a drive system. As the speed of rotation of the turbine is reduced, the annular weight is uncoupled from the power output elements like the turbine. This is effective to avoid some causes of malfunctions of components in clutches and transmission.
A disadvantage to the annular weigh configuration is that the coupling and uncoupling of the weight in the prior art is effected using a centrifugal mechanism, and therefore accurate control of coupling and uncoupling is dependent upon centrifugal forces. Such forces may not provide adequate or desirable control for coupling and uncoupling of the annular weight.